1. Field of the Invention
The present invention relates to the separation of small quantities of isoolefins from streams containing mixtures of an isoolefin and the corresponding normal olefin. The present invention is especially useful for the separation of isobutene from streams containing n-butenes to obtain high purity n-butenes. More particularly, the present invention is useful for removing isobutene from butene-1 containing streams.
2. Prior Art
Isoolefins of 4 carbon atoms are difficult to separate from the corresponding normal olefin by simple fractionation because of the closeness of their boiling points. In the prior art processes that are generally practiced commercially, the isoolefin is selectively absorbed by sulfuric acid and the resulting isoolefin-containing sulfuric acid extract is then diluted and heated or treated with steam to separate the isoolefin.
The n-butenes are required in pure form for homopolymerization and as feeds for the oxidative production of butadiene. Isobutene and diisobutene are of significant value having diverse applications, for example, isobutene is one of the comonomers for butyl rubber and diisobutene is an intermediate in the preparation of detergents. The isobutene oligomers are useful as polymer gasoline. One manner of separating these components is to pass the mixture through a cold acid extraction procedure wherein the stream is fed into a bath of sulfuric acid. Separation is achieved by virtue of the solubility of the isobutene in the sulfuric acid, the n-butenes and other hydrocarbons present passing overhead, for example as shown in U.S. Pat. Nos. 3,546,317 and 3,823,198.
Other processes have used various catalysts for converting the isobutene to diisobutene which is then easily separated from the product stream. For example, a process using a molecular sieve and elevated temperatures is disclosed in U.S. Pat. No. 3,531,539; U.S. Pat. No. 3,518,323 employs a supported nickel oxide catalyst; and U.S. Pat. No. 3,832,418 employs a Group VI or VIII metal deposited on acidic, amorphous silica-alumina in the same manner.
More recently, U.S. Pat. No. 4,215,011 disclosed the use of acid cation exchange resin in a heterogenous combination reaction-distillation system for the selective dimerization of isobutene in the presence of normal butenes. The reaction is highly preferential for the reaction of isobutene with itself although some codimer between n-butenes and isobutene are formed and provides a means to separate isobutene from a C.sub.4 stream.
Although the present process is suited to treat other isoolefin-normal olefins mixtures, it is of particular significance for the recovery of product streams with sufficiently low levels of isobutene to be processable to obtain useable n-butenes; and particularly butene-1, which is the n-butene isomer employed in homopolymerization to produce polybutene; or copolymerization with other monomers; and as a feed for oxidative dehydrogenation to produce butadiene-1,3.
It is a principal feature of the present process that the amount of isobutene in the stream is reduced to levels sufficiently low to allow further separation of a useful butene-1 product. It is another feature of the present process that a very useful product is produced from the isobutene, i.e., polymer gasoline. It is a particular advantage of the present process that it may be operated to obtain the above results with a limited loss of butene-1.
Another feature of the present process is the substantial energy saving over the cold acid method of isobutene removal and a reduction in capital expenditures to replace and/or repair processing equipment that has failed due to the corrosive nature of the sulfuric acid.